Ruby on Rails Better Errors Gem

Better Errors replaces the standard Rails error page with a much better and more useful error page. It is also usable outside of Rails in any Rack app as Rack middleware.

Instead of a plain default error page, Better Errors will display a full interactive stack trace with source code inspection.

If you also include the companion binding_of_caller2 gem into your application Better Errors will be able to also let you inspect local and instance variables and even embed a full REPL into every stack frame of your error page backtrace. Of course you should only ever do that in a development environment.